1. Field of the Invention
The present invention relates to a conductive paste, in particular to a conductive paste suitable for forming electrodes in multilayer electronic components such as multilayer capacitors, multilayer inductors, multilayer actuators and the like, and to a multilayer electronic component manufactured using the conductive paste. In particular, the invention relates to a conductive paste printed directly on ceramic green sheets for forming internal electrodes in multilayer electronic components.
2. Description of the Related Art
Multilayer ceramic electronic components (hereinafter also referred to as “multilayer electronic components”) are usually manufactured as follows. A ceramic green sheet (hereinafter also referred to as “green sheet”) is prepared by dispersing a ceramic raw-material powder such as a dielectric material, a magnetic material, a piezoelectric material or the like in a vehicle containing a resin binder and a solvent. Onto this green sheet a conductive paste for internal electrodes obtained by dispersing, in a vehicle containing a resin binder and a solvent, an inorganic powder comprising as a main component, a conductive powder such as a noble metal, copper, nickel, or the like and optionally a ceramic powder or the like is printed in a predetermined pattern; and the solvent is removed through drying to form thereby an internal electrode dry film. The obtained green sheets having an internal electrode dry film are stacked and pressure-bonded to yield an unfired laminate in which the green sheets are alternately laminated with internal electrode paste layers. After being cut to a predetermined shape, this laminate is fired at a high temperature to simultaneously sinter the ceramic layers and form the internal electrode layers, and yield thereby a ceramic body. A conductive paste for terminal electrodes is subsequently applied to both end faces of the ceramic body using a printing method, a dipping method or the like, and then the paste is fired to yield a multilayer electronic component. The terminal electrode paste and the unfired laminate may also be co-fired.
The growing demand in recent years for miniaturized, highly multilayered electronic components has given strength, in particular, to a trend towards achieving ever thinner ceramic layers and internal electrode layers in multilayer ceramic capacitors using nickel as a conductive powder, which has boosted the use of thin ceramic green sheets.
Butyral resins and/or acrylic resins are generally used as the binder component of ceramic green sheets. On the other hand, cellulose resins, such as ethyl cellulose, are chiefly used as the binder component of conductive pastes. A constitution with such materials is problematic in that, after printing, the highly polar solvents such as terpineol, butyl carbitol, octanol or the like used ordinarily in the conductive paste dissolve the binder of the ceramic green sheet, in a phenomenon called “sheet attack”, which gives rise, among other effects, to deformation and/or insulation loss in the ceramic layers, and is thus highly detrimental to the characteristics of the multilayer electronic product. This problem has become more serious in the wake of the recent trend towards thinner layers in ceramic green sheets, with sheets thinner than 5 μm.
In order to prevent such sheet attack, mixed solvents combining the above highly polar organic solvents with petroleum-derived hydrocarbon solvents, to which the resins in the green sheet have poor solubility, have been conventionally used, as the solvent component of the conductive paste, in an attempt to reduce thereby the proportion of polar solvents (see for instance Patent Document 1). However, increasing the proportion of the above hydrocarbon solvents with a view of curbing sheet attack reduces the solubility of the cellulose resin, which is the binder of the conductive paste, and decreases paste viscosity. In order to maintain an appropriate paste viscosity, it becomes then necessary to use an excess of resin, which increases the thickness of the coated paste layers, thereby causing laminate deformation and so on, which are especially detrimental in highly multilayered laminates, and which impairs the long-term stability of the paste characteristics, thereby giving rise to various difficulties in paste design.
Another known approach for suppressing sheet attack involves using a terpineol derivative such as dihydroterpineol, terpineol acetate or dihydroterpineol acetate as the solvent component of the conductive paste (see for instance Patent Documents 2, 3). However, the effectiveness of, for instance, dihydroterpineol and terpineol acetate is insufficient, while sheet attack cannot be prevented when the green sheet is extremely thin, in particular for thin green sheets having a thickness of less than 5 μm and using a butyral resin as the binder. The use of these solvents results in substantial paste viscosity changes over time, which preclude obtaining a uniform film thickness of the paste layer during printing; thus, multilayer components having excellent electrical characteristics cannot be obtained, since multilayer electronic components manufactured using such pastes with thin green sheets suffer delamination and/or cracking. In particular this is true of highly multilayered components with thinner green sheets.
Other known methods involve forming internal electrodes using a transfer method or the like, without direct printing of a conductive paste on a ceramic green sheet. In such methods, the conductive paste is printed on a carrier film or the like, is dried, and after solvent removal, is transferred onto a ceramic green sheet; although sheet attack is prevented, such methods are technically complex.
[Patent Document 1] Japanese Patent Publication No. 7-240340A
[Patent Document 2] Japanese Patent Publication No. 9-17687A
[Patent Document 3] Japanese Patent Publication No. 7-21833A